Anybody that has driven a manual shifter knows that engine noise is a useful feedback mechanism to the driver.
I ride a motorcycle. Since the engine is out in the open, you can always hear the engine. And yet I find myself relying more on the throttle response and the tachometer to tell me when to shift. They could be entirely silent and I still would shift at the same points.
In fact, I did my final test on a Suzuki LS650 Savage, which does not have a tacho, and I hated it. I had to shift entirely on intuition.